OurBigBook Web email sending

OurBigBook is currently hardcoded to send emails with Sendgrid. That provider was very easy to get started with, and has a free plan suitable for testing. Setup is described at: OurBigBook Web email sending with Sendgrid. Patches supporting other providers in a configurable way are welcome.

In development mode, emails are all logged to the server stdout and not actually sent, unless you run as:

OURBIGBOOK_SEND_EMAIL=1 npm run dev

This can be used to test the email integration locally.

Some research of different methods is shown at: cirosantilli.com/send-free-emails-from-heroku

Related configurations:

after reports that useres had received a "Suspicions link" when clicking the signup link on Gmail, we've tried to follow
and add a TXT record of:
```
google-site-verification=gctCPztssfR8A-fQ_5298gSee_DfFjBj8v9PqAxuhgU
```
to the domain, which Google marked as verified. Let's see if helps.

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